1. Field of the Invention
The present invention relates to a novel composition of high frequency dielectric ceramics. More particularly, the present invention relates to compositions of high frequency dielectric ceramics having high Q values, high dielectric constants, and stable temperature coefficients of the resonant frequency.
2. Description of the Prior Art
Recently, various communication systems using microwave frequencies with a frequency range of 300 MHz to 300 GHz have been developed. These include mobile radio communication systems such as wireless telephone sets, earphones, satellite broadcasting systems, satellite communication systems and the like. Such systems require high frequency dielectric ceramics applicable to resonators, band-pass (or band-stop) filters, duplexers and microwave integrated circuits (MICs) and the like, and demand for such components have greatly increased. High frequency dielectric ceramics to be applied to such communication systems should have the following characteristics: (1) a high dielectric constant, which is required for miniaturization of parts made of dielectric ceramics because the wavelength of microwaves within dielectric ceramics is inversely proportional to the square root of the dielectric loss; (2) a high Q value corresponding to a reciprocal of the dielectric loss, which is required for high performance because dielectric losse is directly proportional to frequency; and (3) a low temperature coefficient of the resonant frequency of a dielectric resonator. See, W. Wersing, "Electronic Ceramics;, B.C.H. Steele ed., p. 67, Elsevier Sci. Pub. Co., New York (1991). In addition, high frequency dielectric ceramics to be applied to such communication systems should have a small change in properties with time, high thermal conductivity and good mechanical strength.
Examples of dielectric ceramics which have been heretofore developed include a Ba(M.sup.+2.sub.1/3,M.sup.+5.sub.2/3)O.sub.3 system wherein M.sup.+2 is Mg or Zn, and M.sup.+5 is Ta or Nb, a Ba.sub.2 Ti.sub.9 O.sub.20 system, and a (Zr,Sn)TiO.sub.4 system. These types of dielectric ceramics have low dielectric loss, while they have dielectric constant less than about 40. See, W. Wersing, supra, and J. Kato, JEE, Sep., pp. 114-118 (1991).
Other examples include a BaO--Sm.sub.2 O.sub.3 --TiO.sub.2 system, a (Ba,Pb)O--Nd.sub.2 O.sub.3 --TiO.sub.2 system, and a (Pb,Ca)ZrO.sub.3 system. These types of dielectric ceramics have dielectric constants as high as about 80, while they have relatively high dielectric losse. For Example, the (Pb,Ca)(Fe,Nb)O.sub.3 system which currently attracts attention in the art, in its optimal composition, has a dielectric constant of about 91 and T.sub.f of +2.2 ppm/.degree. C., while having low Q.sub.x F.sub.0 value of 4,950. See, J. Kato, et al.; Jpn. J. Appl. Phys., Vol. 31, pt. 1, No 9B (1992).
Generally, materials having high dielectric constants exhibit increased dielectric losses and high temperature coefficients of the resonant frequency due to the dipoles and structural defects associated therewith. Furthermore, high dielectric constant materials having high dielectric losses are not suitable for high frequency filters since they lead input signal losses to increase whereby signal transmission is difficult. Therefore, there is a need for a composition which meets the three requirements for high frequency dielectric materials: high dielectric constants, low dielectric losses and stable temperature coefficients of the resonant frequency.
On the other hand, conventional dielectric materials comprising PbO, such as a BaO--PbO--Nd.sub.2 O.sub.3 --TiO.sub.2 system, (Pb,Ca)ZrO.sub.3 system, (Pb,Ca)(Fe,Nb)O.sub.3 system, are usually sintered above about 1300.degree. C. Due to such high sintering temperatures, when the materials are synthesized or sintered, PbO is volatized and consequently, the chemical equilibrium collapses resulting in irregularity of dielectric properties of the materials produced. Therefore, it is desirable to obtain sintered materials which have consistent dielectric properties after sintering.